Certain steels, such as USIBOR™ steels, referred to as aluminized steels because they are coated with aluminum or with an aluminum-based alloy, have very high mechanical properties after hot-drawing and are therefore increasingly used in the field of automobile manufacturing when a weight saving is desired.
Specifically, these steels are designed to be heat treated and then quenched during the hot-drawing operation, the mechanical properties thus obtained enabling a very significant reduction in the weight of the vehicle relative to standard high yield strength steel. These steels are mainly used to manufacture bumper beams, door reinforcements, center pillars, window pillars, etc.
Document EP-A-1878531 suggests welding aluminized steels of this type using a laser/arc hybrid welding process. The laser/arc hybrid welding principle is well known in the prior art.
However, in practice it has been observed that a phase with a lower tensile strength than the base metal and than the weld-metal zone often appears in the welded joint after steel workpieces coated with aluminum or with an aluminum alloy, especially an Al/Si alloy, have been subjected to a hybrid welding operation using a shielding atmosphere formed of a He/Ar mixture and a post-welding heat treatment comprising hot-drawing at 920° C. and then quenching in the tool (at 30° C./s).
Specifically, this lower-tensile-strength phase forms a brittle region in the weld thus obtained, as explained below. These more brittle regions appear in the martensite zone in the form of white-phase islands containing aggregates of aluminum from the surface layer.
Analysis has shown that this phase contains a significant percentage of aluminum (>2%) which prevents austenitic transformation of the steel during its pre-drawing heat treatment, i.e. this phase remains in delta-ferrite form and as a result is softer than the rest of the workpiece, which undergoes a martensitic/bainitic transformation.
During mechanical testing of the joint, after the welding, drawing and subsequent heat treatment, the phase that has not made the martensite transformation may cause the welded joint to crack or even shear because, since these regions of the weld contain aluminum, they are weaker than the deposited metal.
The problem that is faced is how to provide an arc/laser hybrid welding process that improves the mechanical properties of the weld produced in an operation for welding steel workpieces coated with an aluminum-comprising layer. More specifically, the problem is how to obtain a uniform martensite microstructure in the weld-metal zone, i.e. in the weld, after hot-drawing, typically at around 920° C., and quenching in the drawing tool, typically at a cooling rate, between 800° C. and 500° C., of about 30° C./s.